gpgme: respect the doCheck parameter

The current `gpgme` expression ignores the `doCheck` parameter because
upstream's `Makefile` runs the tests automatically as part of the
`buildPhase`.  Let's run the tests as part of the `checkPhase` iff
`doCheck` is set, like the rest of nixpkgs' packages.

In particular, with this commit, `pkgsCross.*.gpgme` will no longer
attempt to run the tests (tests are not supposed to be run when
host!=build).

pkgs/development/libraries/gpgme/default.nix

@@ -110,8 +110,13 @@ stdenv.mkDerivation rec {
     ++ lib.optional stdenv.hostPlatform.is32bit "-D_FILE_OFFSET_BITS=64"
   );
 
+  # prevent tests from being run during the buildPhase
+  makeFlags = [ "tests=" ];
+
   doCheck = true;
 
+  checkFlags = [ "-C" "tests" ];
+
   passthru.tests = {
     python = python3.pkgs.gpgme;
     qt = libsForQt5.qgpgme;